1. Field of the Invention
The present invention relates to an optical microscope of the transmission type for illuminating a sample according to the bright-field illumination method and observing light passing through the sample.
2. Description of the Related Art
Most of the conventional illuminating means for the optical microscope use a halogen lamp of about 20-100W as their light source to uniformly illuminate the sample according to the Koehler's illumination method. The illuminating means of this type comprises the halogen lamp which serves as the light source, a lighting power source for the halogen lamp, and an optical system including a collector lens, a diffusing plate, bright-field and brightness diaphragms and a condenser lens. Light emitted from a filament of the halogen lamp is converted into parallel rays through the optical system to illuminate the surface of the sample.
According to this Koehler's illumination method, the sample can be uniformly illuminated. In addition, the sample cannot be damaged when it is illuminated for a long time because it can be positioned out of the point on which heat emitted from the filament of the halogen lamp is concentrated.
The illuminating means used for the Koehler's illumination method is very complicated and it needs various kinds of optical and mechanical components. This makes the cost of the illuminating means high. Further, the optical components must be adjusted along an optical axis. This also makes the steps of making the illuminating means complicated and its making cost high.
Furthermore, the power source for the halogen lamp needs a power of about 20-100W. Therefore, the microscope can be used only near a commercial power source. When it is to be used outdoors, some accessories such as the generator must be brought together with the microscope.